Recently, the development of display technology leads to advance of thin-film transistors for driving and controlling pixel circuits. In particular, amorphous silicon thin film transistor and low-temperature poly-silicon thin-film transistor have been replaced by metal oxide thin-film transistor. The metal oxide thin-film transistor has many functional advantages in high electron mobility, low open-state current, and fast switching characteristics. Manufacture of the metal oxide thin-film transistors are also advantageous in less characteristic non-uniformity, low cost in material and process, low process temperature, capable of using coating process to form the thin-film transistor with high transmittance rate and large band gap. As a result, the metal oxide thin-film transistor has been used in many display applications that need fast response time and large current, such as high-frequency response, high resolution, large-size liquid crystal display and organic light emitting display.
The state-of-art metal oxide thin-film transistor, however, is not stable in performance. When the gate of the metal oxide thin-film transistor is under a low voltage level for relatively long time, the metal oxide thin-film transistor will suffer a negative bias stress, leading to a negative shift of transistor threshold voltage level. As a result, various abnormal display phenomena occur in the images displayed by the display apparatus based on the metal oxide thin-film transistors.